In the field of electric motors it is well known that brushless designs are advantageous (See the book, D.C. Motors, Speed Controls and Servo Systems, Chapter 6, Electro-Craft Corporation, Hopkins, MN (1980). Such motors typically employ Hall effect magnetic sensors, together with permanent magnets suitably attached to the motor shaft, to provide commutation information for the motor windings. These widely used one dimensional rotary motor designs have also been extended to one dimensional linear applications using either magnetic or optical position sensors, as shown in U.S. Pat. No. 4,509,001 to N. Wakabayashi et al (April 2, 1985) or in U.S. Pat. No. 4,618,808 to J. Ish-Shalom (October 21, 1986). In all such cases the position sensor is an additional element, rigidly attached to the motor itself and accurately positioned with respect to the motor armature. It must be particularly noted in addition that no brushless DC motor in the prior art can move freely through arbitrarily large distances in more that one dimension.
On the other hand there exists a different class of motor, namely stepping motors (See the book by Vincent Del Toro, Electric Machines and Power Systems, p. 433ff, Prentice Hall, Englewood Cliffs, N.J. (1985). Such motors are open-loop devices in which the motor position advances incrementally in step with sequential electrical driving signals. Motors of this type have been extended to provide full two dimensional motion on planar surfaces over arbitrary distances, as shown in U.S. Pat. No. Re. 27,436 to B. A. Sawyer (July 18, 1972). Some efforts have also been made to produce a true two-dimensional brushless DC motor from such stepping motors by attempting to measure the motor position magnetically (but now in two dimensions) and again using this information to commutate the motor windings. However such schemes to date have turned out to be both difficult to implement and complex (U.S. Pat. No. 3,857,078 to B. A. Sawyer (Dec. 24, 1974)) and no commercally viable motor of this type has yet been reported. One of the key difficulties in such attempts is that of determining the two dimensional commutation (position) information rapidly and accurately and in such a way that motion in one direction does not disturb the position information available in the perpendicular direction.
There does, however, exist a different scheme for position determination in two dimensions which turns out to be well suited to this task. This is by measurement of the capacitance between a suitably designed electrode array and an appropriately patterned two dimensional surface. (See my U.S. Pat. No. 4,893,071, issued on January 9, 1990.)
It is an object of the current invention that this capacitive position-sensing scheme be applied to both one and two dimensional actuators.
A further inherent problem that arises in variable reluctance motors using unidirectional driving currents is that of dissipating the stored magnetic energy that is present in each winding at the turn-off instant. This problem is exacerbated if the motor uses a fine magnetic pitch and moves at high speed, thereby necessitating rapid inductor switching action.
While energy-recovery schemes for variable reluctance motors exist, some of the prior approaches require separate multiple windings on the motor armature to achieve the intended result. See, for example, "Variable Reluctance Motors for Electric Vechicles," NASA TECH. BRIEF, Vol. II, No. 10, Item 113, JPL Invention Report NOP-16993ISC-1444, J. H. Lang and N. L. Chalfin (Dec., 1987.) The separate motor windings impair other qualities of the motor.
A subsidiary objective of the current invention to arrange for both a speed up of the turn-off process and a corresponding speed up of the turn-on process in the next motor winding, further to arrange that the collapsing magnetic field in one motor winding uses the back-emf thereby generated to temporarily increase the voltage available to the next motor winding to be switched on, and furthermore, to achieve all the foregoing without extra motor windings and to apply all of the foregoing to reversible three phase motors, unlike those of U.S. Pat. No. 3,486,096 to G. W. Van Cleave (Dec,. 23, 1969) and U.S. Pat. No. 4,533,861 to J. E. Rogers, et al (Aug. 6, 1985), existing in the prior art that are also intended to provide rapid magnetic field switching in stepping motors by use of the back emf effect. These latter two prior schemes are unable to handle three phase motors including direction reversal.